This invention relates generally to an improved power control arrangement for a surface heating unit in a cooking appliance such as a domestic electric range, and more particularly to a control arrangement which includes a temperature sensor responsive to the temperature of a utensil resting on the surface heating unit.
Various surface unit power control arrangements utilizing a utensil temperature sensing device have been developed in the past, some of which are presently in use in commercially available domestic ranges. Typically, such systems generally referred to as automatic surface units, include a temperature sensing device such as a bimetallic device or thermistor device mounted so as to be in thermal contact with the utensil. Current to the heating elerent is controlled as a function of the state of the sensing device. When the sensed temperature is less than a predetermined threshold temperature set by user manipulation of input control knobs or switches mechanically coupled to the sensor device and operative to vary the threshold as a function of user input, the heating element is energized at full power, and when the temperature exceeds the threshold the heating element is de-energized. Such electromechanical sensing and control devices, due at least in part to the tolerance buildup, are inherently inaccurate both in temperature sensing and in controlling power in response to the sensed temperature. The latter problem results, at least in part, from an inability of such controls to provide small duty cycle changes, particularly at low duty cycles, i.e. duty cycles less than 50%. Consequently, the temperature control performance of such controls is characterized by undesirably large overshoots and undershoots about an inaccurate nominal temperature.
In view of the above-described shortcomings of automatic surface unit control arrangements known in the art, a control arrangement which accurately senses utensil temperature and which provides relatively tight temperature control with minimal temperature overshoot would be highly desirable.
It is therefore an object of the present invention to provide an improved power control arrangement for an automatic surface unit which provides reliable temperature control by accurately measuring utensil temperature and which provides a rapid thermal response to changes in heat setting selections or changes in utensil load affecting temperature in both Boil and Fry operating Modes, while limiting the temperature overshoot and undershoot.
It is a further object of the present invention to provide a power control arrangement of the aforementioned type which incorporates an electronic controller to implement a plurality of operating modes to provide accurate temperature control and fast thermal response for a variety of different loads and cooking objectives.